The present invention relates generally to a method of and an apparatus for optically recording and reproducing information onto and from an optical recording medium such as an optical card, and more particularly to improved control for allowing a light beam, irradiated from an optical head, to access a desired track on the information recording surface of an optical recording medium.
Optical recording and reproducing apparatus are known which record and reproduce information onto and from a card-shaped optical recording medium (hereinafter referred to as an optical card) by moving the optical card relative to the optical axis of a laser beam. With the developments and wide spread use of computers etc., a wide use of the optical card has been strongly hoped for in recent years because it is highly portable and safe and yet provides a relatively large storage capacity for its small size. Thus, a variety of applications of the optical card have been proposed, among which is an application as a medium for recording patients' diagnosis in medical organizations.
A typical structural example of the known optical card is shown in FIGS. 4 and 5, of which FIG. 4 is a plan view of the known optical card 11 and FIG. 5 shows, in enlarged scale, a section "A" of the optical card 11 of FIG. 4. In these figures, reference numeral 12 denotes a recording/reproducing area, 13 denotes guide tracks, and 14 denotes a data track. On the recording/reproducing area 12 is formed a recording layer that is for example made of silver chloride photographic material as the base material. By irradiating a laser light spot of a suitable energy level from an optical head onto the recording layer, an optical information unit called a "pit" is formed in the data track 14. The position of the irradiated laser light spot on the recording layer is variable by moving the optical card 11 relative to the optical head in the X-axis direction (direction parallel to or along the length of the data and guide tracks of the optical card 11), so that a series of pits can be formed in a desired arrangement corresponding to desired digital information. Thus, recording and reproduction of desired digital information are performed by writing and reading the pit rows onto and from the recording layer of the optical card 11.
In such a case, in order to form pit rows in the data track 14 of the optical card 11, such an approach is generally employed which uses a drive mechanism such as a linear motor to move the optical card 11 relative to the optical head. However, due to a limited operational accuracy of the drive mechanism, this prior approach can not prevent occurrence of mechanical position errors, due to which pits can not be formed accurately in the middle of the data track 14 located between the guide tracks 13. This presents the significant problem that desired information can not be recorded or reproduced accurately.
In order to avoid the above-mentioned problem, it is absolutely necessary to perform the pit recording and reproduction with the laser beam spot accurately positioned in the middle between the two guide tracks 13. To this end, automatic tracking control (often abbreviated "AT control") has been conventionally employed in an attempt to compensate for any mechanical position error caused.
This automatic tracking control is generally performed on the basis of the so-called "three-beam method", in accordance with which three laser beams spaced apart from each other by a predetermined distance are irradiated from the optical head in such a manner that the central laser beam corresponds to the data track 14 as a read/write beam and the two laser beams on both sides of the central beam (side laser beams) correspond to the guide tracks 13 on both sides of the data track 14 as tracking beams. Namely, the three-beam method measures the respective reflected lights of the two side laser beams from the optical card 11 so as to servo-control the irradiated beam spot positions in such a manner that the tracking beams accurately correspond to the guide tracks 13 in predetermined positional relations thereto and thus the central read/write beam is allowed to always be accurately positioned in a predetermined middle part of the data track 14. Further, it is necessary to have the laser light beam constantly stably focused on the recording layer of the optical card 11, automatic focusing control has conventionally been performed for this purpose.
The above-mentioned automatic tracking and focusing control operations are performed by minutely driving the objective lens of the optical head, via electromagnetic force applied via a tracking coil and a focusing coil, respectively, in the Y-axis direction (i.e., direction transverse to the data and guide tracks of the optical card 11) and in the Z-axis direction (i.e., direction perpendicular to the recording/reproducing surface of the optical card 11). The objective lens serves to focus the laser beam irradiated from the optical head onto the recording layer of the optical card 11 so as to form a focused light spot (three light spots in the case where the above-mentioned three-beam method is employed) on the recording layer.
In the art, such control is also known which is intended for allowing the light beam to access a desired target track or the vicinity thereof by moving the light beam spot across only one or several tracks (in the Y-axis direction) relative to the optical card 11. Where the current position of the laser beam spot is only one or several tracks away from the target track, the control is performed by only moving the objective lens in the Y-axis direction while the body of the optical head is fixed in position, and this control is called a "near-jump" control. On the other hand, where the current position of the laser beam spot is relatively many tracks away from the target track, the light bean spot is positioned in the vicinity of the target track by rapidly moving the body of the optical head itself in the Y-axis direction. This control is called a "far-jump" control. The above-mentioned automatic tracking control is maintained inactive (OFF) during execution of either the near-jump control or the far-jump control, and upon completion of the near-jump or far-jump control, the automatic tracking control is turned ON to draw the light beam spot onto the target track.
As noted above, the information recording or reproducing operation with respect to the optical card is performed while moving the optical card in a parallel direction to the tracks (in the X-axis direction). This movement of the optical card in the X-axis direction is a reciprocating movement where the movement direction is reversed upon the card reaching one of predetermined opposite limit points. When the movement direction of the card is reversed, abrupt deceleration and acceleration would take place, causing considerable mechanical vibration. If the above-mentioned jump control is performed for accessing a desired track while such mechanical vibration is present, there would be caused the operational errors, due to which the light beam spot is drawn to a wrong track when the automatic tracking control is turned ON. One of the conventionally-known approaches for avoiding such operational errors is to perform the jump control for accessing the desired track when the optical card is moving at constant speed, instead of during the deceleration and acceleration of the optical card. But, in that case, there would arise the problem that a part of a storage area corresponding to the constant-speed movement can not be used for information storage purposes, and hence the storage capacity is greatly limited.
In an attempt to provide a solution to the problem, Japanese Patent Laid-open Publication No. SHO 64-27028 (corresponding to U.S. Pat. No. 4,982,391) proposes that, when the movement of the optical card in the X-axis direction is to be reversed, the optical card be prevented from moving over a predetermined time zone and the jump control be performed, during this stop time zone, for accessing a desired track. An example of such control is shown in FIG. 6A, which shows a time-variation instruction value for the X-axis direction movement speed of the optical card, and FIG. 6B, which shows the time zone during which the jump control is performed. In FIG. 6A signs "+" and "-" of the instruction speed values correspond to plus and minus in the X-axis movement direction of the optical card. In this example, the stop time zone comprises time sections T1, T2 and T3. the first time section T1 is set at a specific time length for allowing the mechanical vibration due to the abrupt deceleration to sufficiently lower down, the second time section T2 is at a time length necessary for the jump control, and the third time section T3 is a settling time immediately after the automatic tracking control loop is turned ON upon completion of the jump control, so to speak, a "wait time" before the acceleration of the optical card is started.
However, in the above-discussed prior art, because the deceleration control is still abrupt, it is necessary to set the time section T1 to be sufficient for decreasing the mechanical vibration resultant from the abrupt deceleration, thus requiring extra operation time. Consequently, the prior art has the problem that it requires extra time to access a desired target track for recording or reproduction of information.